The invention relates to a process for the dissolution of salts, especially catalytically active sodium chloride NaCl and ferric chloride FeCl3 in liquid 1,2 dichloroethane, hereinafter referred to as “EDC”, which primarily serves as an intermediate product in the production of monomer vinyl chloride, hereinafter referred to as “VCM”, which subsequently is used to produce polyvinyl chloride (PVC) by ultrasonic treatment. When EDC reacts to form VCM hydrogen chloride HCl is obtained. Hence, EDC is preferably produced from ethene C2H4 and chlorine Cl2 in such a way that an equilibrated balance is maintained between the hydrogen chloride (HCl) produced and consumed in the various reactions, which is substantiated by the following reaction equations:Cl2+C2H4→C2H4Cl2(EDC)+180 kJ/Mol  (1)C2H4Cl2(EDC)→C2H3Cl(VCM)+HCl−71 kJ/Mol  (2)C2H4+2HCl+½O2→C2H4Cl2(EDC)+H2O+238 kJ/Mol  (3)
Reaction (1) also referred to as direct chlorination is usually carried out as gas/liquid interfacial reaction in loop reactors of various designs, which provide for the dissolution of ethene C2H4 and thus for the speed-relevant criterion, or it takes place in the liquid phase. The EDC is applied as the solvent used for the dissolution of the reactants. Many years of practical experience have shown that the purity of the EDC produced is of major importance for the cost effectiveness and the final product purity which can be achieved by reaction (2) and consequently for the complete process. This gave way to many attempts to minimise the side reactions related to reaction (1) as, for example, by the development of efficient catalyst systems.
Reaction (1) is of the “addition type”, in which generally metal halides with Lewis acid properties and a metal halide of a metal that belongs to the first main group of the periodic table are used as catalyst as well as other organic catalysts if necessary. NL-A 6901398 provides for the use of ferric chloride FeCl3, sodium chloride NaCl and lithium chloride LiCl as preferred metal halides. DE 41 03 281 describes the use of a mixture of ferric chloride FeCl3 and sodium chloride NaCl at a molar ratio ranging from 1 to 1.5 and 1 to 2. DE 43 18 609, in contrast to this, outlines that the reaction is especially efficient if the molar ratio of NaCl and FeCl3 is kept below 0.5. In these cases, sodium chloride NaCl reacts with ferric chloride FeCl3 in solution by reaction (4), thus forming sodium tetrachloroferrate NaFeCl4 which has been known to be an efficient catalyst for a long time:Na+Cl−+FeCl3→Na+FeCl4−  (4)
In this context, however, it is problematic that sodium chloride NaCl is hardly soluble in EDC. This also applies to the fact that under normal reaction conditions and in the presence of minor amounts of water, ferric chloride FeCl3 tends to convert to hydrogen tetrachloroferrate which is known for its highly corrosive potential as described in EP 00 82 342. Such a corrosive potential can be suppressed by adding sodium chloride NaCl in a stoichiometric surplus referred to reaction with ferric chloride FeCl3 according to the equation (4) so that it becomes desirable to dissolve as much sodium chloride NaCl as possible. If, however, solid sodium chloride NaCl is added to the reaction loop, solute may easily form in sections with poor flow and will not dissolve even under normal operating conditions of the direct chlorination unit and thus give way to side reactions.
As described in patent DE 25 40 291 further difficulties may also be caused by clogging as a result of inefficient solution of the sodium chloride NaCl. A specific attempt has also been described, i.e. applying instead anhydrous sodium tetrachloroferrate NaFeCl4 which according to equation (4) consists of the sum of sodium chloride NaCl and ferric chloride FeCl3 to be dissolved in EDC because its solubility is better than that of its components. In this case, however, the technological problem is shifted to the production of sodium tetrachloroferrate NaFeCl4 which is very expensive according to U.S. Pat. No. 3,729,543.
Various attempts in the past have shown that methods to produce the preferred catalyst system are very sophisticated because the catalysts can be dissolved in the EDC reaction system with great difficulty only. DE 44 25 872 describes an example on the basis of a solution of only 170 ppm sodium chloride NaCl and 780 ppm ferric chloride FeCl3.